The present invention generally relates to a scanner, and more particularly, to an optical scanning device to be used, for example, in a laser printer, bar code scanner or the like.
In recent years, as office automation appliances, etc. have come to be widely used, attention has been directed to a laser printer with a high accuracy and a low noise as one type of output device thereof. Although such a laser printer currently in use employs a polygonal mirror scanner as an optical scanning device in many cases, development of holographic scanners which can be readily produced on a large scale has also been under way for achieving reduction in cost, as disclosed in detail, for example, in Japanese Patent Laid-Open Publications Tokkaisho Nos. 62-28708, and 62-234117, etc.
One example of the conventional optical scanner employing the holographic scanner as referred to above will be explained hereinafter with reference to FIG. 5 showing general construction thereof.
In FIG. 5, the known optical scanning device P includes a semi-conductor laser unit 1, a holographic lens 2, a holographic disk 3, and a motor 4 attached to the holographic disk 3.
In the above arrangement, laser light emitted from the semi-conductor laser unit 1 is formed into a converged laser beam 5. On the holographic disk 3 disposed on an optical path of the converged laser beam 5, a plurality of transmission type holograms (not particularly shown) are provided in an annular shape, and by the lens action of such transmission type holograms, the converged laser beam 5 is focused at a focal point of said transmission type hologram. Since focal point of the transmission type holograms vary varies through rotation of the motor 4 associated therewith, the converged laser beam 5 must be subjected to scanning on a photoreceptor drum 6.
The known arrangement as described so far, however, has a problem that, since the transmission type hologram employed therein is formed by laser interference exposure, with a resist sensitive to the laser beam being applied on a surface of a transparent substrate, the resist forming the interference pattern tends to be swollen by absorbing moisture from the air so as to produce turbulence in the pattern, thus resulting in undesirable variation in the scanning characteristics.
Another disadvantage is such that, the hologram generally has a low diffraction efficiency, and as compared with the transmission efficiency of normal lenses, it has very poor efficiency for utilizing light. Moreover, in the arrangement as described above, there has also been a problem that, since two sheets of holograms, i.e. the hologram lens and the hologram disk are employed, the light utilizing efficiency is still further lowered.